Re: [PATCH, RFC] Add public function syscall_no_errno

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 28/01/24 13:39, Askar Safin wrote:
> Hi! I want glibc to have function "syscall_no_errno" on Linux.
> It should do the same "syscall" does, but it should not interpret
> return value and set errno. This is useful for calling syscalls
> such as getuid. I. e. now the user can call directly all syscalls
> including getuid and similar.
> 
> I add example patch. It is quick-and-dirty. I was unable to figure out
> how to add function to headers. So, please, don't apply it as-is.
> 
> I just want to know do you agree with my proposal. If yes, I will try
> to write better patch.
> 
> I will repeat: currently glibc is simply incomplete, because it
> does not provide a way to call directly syscalls, such as getuid.
> So the user have to craft assembly, which is very difficult.

Indeed there some old syscalls where trying to issue them directly with
syscall is problematic (like 'time' and 'brk' for some ABIs), but getuid
is not one of them.  Also, recent Linux kABI is trying to avoid such 
problematic interfaces to return the value as the return code and make
the invalid value similar to all cases.  So these are not very compeling
reason to add a non-standard symbol to issue syscalls.

CCing Rich, maybe he has a different view about this.

> 
> The patch is against current master, i. e. ae49a7b29acc184b03c2a6bd6ac01b5e08efd54f
> 
> --
> 
> diff --git a/misc/Versions b/misc/Versions
> index d5b348e8..ad37a4c2 100644
> --- a/misc/Versions
> +++ b/misc/Versions
> @@ -71,7 +71,7 @@ libc {
>      # s*
>      sbrk; select; setdomainname; setfsent; sethostent; sethostid; sethostname;
>      setlogmask; setmntent; setregid; setreuid; setttyent; setusershell; sstk;
> -    stty; sync; syscall; syslog;
> +    stty; sync; syscall; syslog; syscall_no_errno;
>  
>      # t*
>      tdelete; tfind; truncate; tsearch; ttyslot; twalk;
> diff --git a/posix/unistd.h b/posix/unistd.h
> index 54d7d752..2f0f6e79 100644
> --- a/posix/unistd.h
> +++ b/posix/unistd.h
> @@ -1089,6 +1089,7 @@ extern void *sbrk (intptr_t __delta) __THROW;
>     In Mach, all system calls take normal arguments and always return an
>     error code (zero for success).  */
>  extern long int syscall (long int __sysno, ...) __THROW;
> +extern long int syscall_no_errno (long int __sysno, ...) __THROW;
>  
>  #endif	/* Use misc.  */
>  
> diff --git a/sysdeps/unix/sysv/linux/syscall-names.list b/sysdeps/unix/sysv/linux/syscall-names.list
> index aac065e7..dac4d78e 100644
> --- a/sysdeps/unix/sysv/linux/syscall-names.list
> +++ b/sysdeps/unix/sysv/linux/syscall-names.list
> @@ -612,6 +612,7 @@ sys_epoll_create
>  sys_epoll_ctl
>  sys_epoll_wait
>  syscall
> +syscall_no_errno
>  sysfs
>  sysinfo
>  syslog
> diff --git a/sysdeps/unix/sysv/linux/syscall.c b/sysdeps/unix/sysv/linux/syscall.c
> index 3cff1d97..481b18a4 100644
> --- a/sysdeps/unix/sysv/linux/syscall.c
> +++ b/sysdeps/unix/sysv/linux/syscall.c
> @@ -41,3 +41,20 @@ syscall (long int number, ...)
>      }
>    return r;
>  }
> +long int
> +syscall_no_errno (long int number, ...)
> +{
> +  va_list args;
> +
> +  va_start (args, number);
> +  long int a0 = va_arg (args, long int);
> +  long int a1 = va_arg (args, long int);
> +  long int a2 = va_arg (args, long int);
> +  long int a3 = va_arg (args, long int);
> +  long int a4 = va_arg (args, long int);
> +  long int a5 = va_arg (args, long int);
> +  va_end (args);
> +
> +  long int r = INTERNAL_SYSCALL_NCS_CALL (number, a0, a1, a2, a3, a4, a5);
> +  return r;
> +}
> diff --git a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> index aea7848e..55d9dadd 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> @@ -1761,6 +1761,7 @@ GLIBC_2.2.5 sys_nerr D 0x4
>  GLIBC_2.2.5 sys_sigabbrev D 0x200
>  GLIBC_2.2.5 sys_siglist D 0x200
>  GLIBC_2.2.5 syscall F
> +GLIBC_2.2.5 syscall_no_errno F
>  GLIBC_2.2.5 sysconf F
>  GLIBC_2.2.5 sysctl F
>  GLIBC_2.2.5 sysinfo F
> diff --git a/sysdeps/unix/sysv/linux/x86_64/syscall.S b/sysdeps/unix/sysv/linux/x86_64/syscall.S
> index 43af8087..04483251 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/syscall.S
> +++ b/sysdeps/unix/sysv/linux/x86_64/syscall.S
> @@ -26,6 +26,18 @@
>  
>  
>  	.text
> +ENTRY (syscall_no_errno)
> +	movq %rdi, %rax		/* Syscall number -> rax.  */
> +	movq %rsi, %rdi		/* shift arg1 - arg5.  */
> +	movq %rdx, %rsi
> +	movq %rcx, %rdx
> +	movq %r8, %r10
> +	movq %r9, %r8
> +	movq 8(%rsp),%r9	/* arg6 is on the stack.  */
> +	syscall			/* Do the system call.  */
> +	ret			/* Return to caller.  */
> +
> +PSEUDO_END_NOERRNO (syscall_no_errno)
>  ENTRY (syscall)
>  	movq %rdi, %rax		/* Syscall number -> rax.  */
>  	movq %rsi, %rdi		/* shift arg1 - arg5.  */